Sewage sludge and sludge from other sources frequently must be dewatered for further processing, e.g. drying prior to incineration, for composting or for disposal. One of the dewatering methods used is the pressing of the sludge in a dewatering press to form a sludge cake, a sludge volume being thereby compressed to express the water therefrom.
Some sludges have been characterized as difficult to dewater and include sewage sludges derived from biological water and sewage treatment plants. Other difficult to dewater sludges include so-called hydroxide sludges, sludge containing fats and protein components, e.g. as derived from food processing waste-water treatment, and sludges which are formed in electro-mechanical processes, i.e. so-called galvanotechnical sludges.
A characteristic of difficult to dewater sludges is that the interstitial passages of the sludge volume, through which the water must drain upon pressing, tend to block or plug at an early stage in the pressing so that even a substantial increase in the pressing pressure will not express the desired amount of water from the sludge.
To overcome this drawback, it is known to provide dewatering auxiliaries or agents, for example so-called drainage agents, in the sludge volume. These drainage agents may be granular substances or short fibers, for example short segments of monofilament synthetic resin fibers, which are mixed into the sludge volume, are statistically distributed therein and are not connected to the sludge dewatering press. These drainage agents maintain interstitial channels in the sludge volume during compression.
Since the drainage agents are consumable, they add considerably to the cost of the process and, because they must be disposed of with the filter cake, increase the volume of material which must be handled for disposal. This is particularly a problem when a drainage agent includes short segments of plastic fibers which may not readily decompose in a land fill.
Analysis of the dewatering process has shown that the drainage agents form uniform passages or micropassages throughout the filter volume even at high pressures, enabling the expressed water to run off. Nevertheless, the degree of dewatering is unsatisfactory, and it has been found that when high dewatering pressures of, for example, 10 bar and higher are used, there are still limits in the amount of water which can be expressed so that the use of drainage agent particles in the sludge volume does not solve the problem fully.